Wash, clean and slice the aubergines lengthwise. Spread on kitchen paper and sprinkle with salt. Let stand for 15 minutes. Wash and clean the zucchini and also cut them lengthwise into slices. Wash the thyme, dab dry, put some aside for garnishing, chop the rest finely. Dab aubergines dry with kitchen paper.
Heat the oil in portions in a large frying pan, first fry the aubergines then the courgettes. Sprinkle aubergines and courgettes with pepper and lemon thyme, season courgettes with salt.
Melt the fat in a pot, sprinkle flour over it and sauté while stirring. Gradually add 300 ml water and milk while stirring. Bring to the boil, stir in stock and simmer for 5 minutes. Season sauce with salt, pepper, lemon peel and juice. Drain the mozzarella, cut 300 g into slices.
Pour some sauce into a greased casserole dish and spread. Place 3 lasagne plates on top. Place about 1/3 of the aubergines, courgette slices, sauce and half of the mozzarella on top, one after the other. Cover with 3 lasagne plates. Once again layer about 1/3 of the vegetables, sauce and the remaining cheese on top. Cover with the rest of the lasagne sheets, add the remaining vegetables and sauce. Coarsely grate 100 g mozzarella, mix with parmesan and sprinkle over the casserole. Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 25-35 minutes. Garnish with thyme